CEMENT-BASED MATERIALS IN CONTACT WITH WATER REACTIVE TRANSPORT DRIVEN BY SURFACE PROCESSES

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1 CEMENT-BASED MATERIALS IN CONTACT WITH WATER REACTIVE TRANSPORT DRIVEN BY SURFACE PROCESSES M. Schwotzer 1, T. Scherer 2, A. Gerdes 1,3 Kompetenzen 1 Forschungszentrum und Karlsruhe, Visionen ITC-WGT des ITC-WGT 2 Forschungszentrum Karlsruhe, INT 3 University of Applied Sciences-Karlsruhe 1 M. Schwotzer, T. Scherer, A. Gerdes

2 Outline Deterioration of cement based materials in tap water storage Experimental investigation of reaction mechanims Results Model of the reaction mechanism Validation on case studies Conclusions 2 M. Schwotzer, T. Scherer, A. Gerdes

3 Observations in tap water reservoirs Deterioration of cement-based materials during service life in contact with hard tap water (in carbonic acid equilibra) Appearance: brown spots strictly local sometimes arranged in regular patterns Mechanical properties: loss of strength 3 M. Schwotzer, T. Scherer, A. Gerdes

4 Observations in tap water reservoirs Chemistry: discharge of Ca(OH) 2 intrusion of CaCO 3 degradation of CSH Kinetics: Sometimes 6 months after application in permanent contact with hard tap water: lateral- and depth-expansion of the damage Electrical fields? Microbiology? Acidcleaningagents? Hydraulic changes during tap water reservoir use? 4 M. Schwotzer, T. Scherer, A. Gerdes

5 Open questions Reactive transport: Transport: Diffusion Chemical reactions: dissolution precipitation Why does the damage occur with cement-based materials in permanent contact with (hard) tap water? What about a protection against leaching by CaCO 3 -layers? Why do we observe such significant local differences in the velocity of reactive transport? 5 M. Schwotzer, T. Scherer, A. Gerdes

6 Experimental program Focus Experiment Chemical reactions Methods Material/water Interface Material/water Interface Exposition of (fresh cut) cement paste surfaces Transport processes Exposition cement paste samples XRD, ESEM-EDX, FIB-ESB TG, MIP, ESEM-EDX Material properties Material properties Acclelerated transport by electrical fields influence of the pore structure on reactive transport processes Exposition experiments with commercial available mortars TG, XRF MIP ESEM, TG Validation by case studies Characterisation of chemistry, mineralogy and structural properties (e.g. porosity and pore size distrib.) in case studies XRD, TG, ESEM, FIB MIP 6 M. Schwotzer, T. Scherer, A. Gerdes

7 Cement-based materials in tap water reservoirs Construction phase: high humidity (~100%) low temperature (~10 C) Î condensate formation can occur Utilization: permanent in contact with tap water filling and emptying 7 M. Schwotzer, T. Scherer, A. Gerdes

8 Experimental setup Experimental setup to characterize the cement paste/ water interaction, based on practical frame conditions Fresh cut slices (1 mm) of white cement paste (w/c=0,4) Temperature = 11 C Different types of exposition: Water demineralized water hard drinking water Basic conditions continuous water exchange (V=25 L) stirred (V=0,5 L), open system 8 M. Schwotzer, T. Scherer, A. Gerdes

9 What happens at the material/water interface? Focused Ion Beam cuts Contact with hard tap water: development of a covering CaCO 3 -layer no significant impact on pore structure and chemical composition 9 M. Schwotzer, T. Scherer, A. Gerdes Contact with demineralized water: No dense covering CaCO 3 -crystallization Chemical attack on the cement paste leaching enlargement of the pore structure

10 Calcite and portlandite content vs. time 10 M. Schwotzer, T. Scherer, A. Gerdes

11 Porosity and pore size distribution Hard tap water: no significant change in porosity and pore size distribution Demineralized water: Increase of porosity and changes in pore size distribution Increase in the smallest pore volumes indicates that the paste is changing in ways other than just being destroyed, which would tend to decrease the surface area /fine structure. (Thomas et al., 2004) THOMAS J. J., CHEN J. J., ALLEN A. J., JENNINGS H. M. (2004): Effects of the decalcification on the microstructure and surface area of cement and tricalcium silicate pastes, Cem. Concr. Res., 34, M. Schwotzer, T. Scherer, A. Gerdes

12 Surface processes in hard tap water Cont. water exchange (25 L reservoir), tap Water: HCO 3- available 12 M. Schwotzer, T. Scherer, A. Gerdes

13 Reaction with demineralized water Demineralized water: Ca-leaching from the CSH-gel is associated with a decrease of volume Decalcification shrinkage (Chen et al., 2006) leads to the formation of micro-cracks in the surface zone of the material In the stirred reactor: CaCO 3 overgrows the leaching features CHEN J. J., THOMAS J. J., JENNINGS H. M. (2006): Decalcification shrinkage of cement paste, Cem. Concr. Res., 36, M. Schwotzer, T. Scherer, A. Gerdes

14 Reaction with demineralized water Cont. water exch. (25 L reservoir), demineralized Water: low CO 2 availability Stirred reactor (0,5 L) good CO 2 availability growth of thick CaCO 3 -layers irregular crystallisation 14 M. Schwotzer, T. Scherer, A. Gerdes

15 Step 1: Initial damage - material-condensate interaction Local reaction under a condensate water drop: 15 M. Schwotzer, T. Scherer, A. Gerdes

16 In this case, the CaCO 3 -layer has no efficient function as barrier against transport processes! WPC paste w/c=0.4, 10 days stirred in demineralized water (0.5 L reactor) 16 M. Schwotzer, T. Scherer, A. Gerdes

17 Step 2: further reaction in contact with hard tap water Consequence: lateral- and depthexpansion of the damaged areas 17 M. Schwotzer, T. Scherer, A. Gerdes

18 Summary of the reaction mechanism (1) Inital damage by reaction with condensate solutionof CO 2 of in the condensate Surface crystallisation triggers further reactive transport Structural changes support the progression of the reaction in deeper areas (2) Progress of the damage during contact with hard tap water Crystallisation front progresses in the depth Decalcification shrinkage promotes the lateral developement of the damage 18 M. Schwotzer, T. Scherer, A. Gerdes

19 Case Study: Interface material/water What happens in the surface near µ-meters? Focused Ion Beam preparation of a deteriorated area (white) and an intact area (gray) 19 M. Schwotzer, T. Scherer, A. Gerdes

20 Case study: Interface material/water EDX-analysis on FIB-cuts in the surface of a damaged and a intact mortar surface Structure of CaCO 3 layer Irregular in the damaged area Ca/Si ratio In the intact area, the CaCO 3 layerisonlyfewµm thick In damaged area, the formation of CaCO 3 has reached deeper regions of the material 20 M. Schwotzer, T. Scherer, A. Gerdes

21 Conclusions The formation of a CaCO 3 -layer can play a protective role fast crystallisation; Ca 2+ and HCO 3- from the water destructive role crystal growth affects the pore water chemistry If Ca 2+ is supplied by the pore solution, CaCO 3 formation triggers a further chemical degradation of the solid phases of the cement paste Leaching but also CaCO 3 formation causes microstructural changes in the CSH also by decalcifiction shrinkage An initial leaching can be crucial for the long-term behaviour of cement based materials in permanent contact with water (also if the water is considered to be not agressive ) favorable conditions for CaCO 3 precipitation are shiftet in deeper areas of the material subsequent ingress of HCO 3- and Ca 2+ can occur only by diffusion slow process formation of a dense coating unlikely The sturctural properties of a CaCO 3 layer are determined by frame conditions (e.g. watercomposition, temperature) affecting the efficency as a diffusion barrier 21 M. Schwotzer, T. Scherer, A. Gerdes

22 Thank you for your kind attention ~ 20 µm 22 M. Schwotzer, T. Scherer, A. Gerdes

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27 XRD of the sample surfaces 27 M. Schwotzer, T. Scherer, A. Gerdes

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